1. Field
The present disclosure relates to circuits and systems for wireless communications, in particular, to a two-step channel selection method comprising a coarse and a fine channel selection for wireless transceiver front-ends in acquisition of channels from different standards.
2. Related Art
In addition to the aggressive requests of high integration and low-power dissipation, multistandard compliance is an important feature of emerging wireless transceiver integrated circuits (ICs) to allow seamless interswitch one terminal through diverse cellular and wireless-network communication standards.
Prospective receiver analog front-ends (AFEs) for attaining those requirements are presently restricted to zero-intermediate frequency (zero-IF) and low-IF architectures, whose operating principles can be pictorially described by the complex-signal spectral-flow (CSSF) illustrations in FIGS. 1(a) and (b), respectively. The basic components include a mixer, a frequency synthesizer (FS), a local oscillator with in-phase (I) and quadrature (Q) outputs (I/Q-LO), a channel-select filter (CSF), and an analog-to-digital converter (A/D). Off-chip and power-hungry image-reject filters, strongly required in superheterodyne architectures, are no longer necessitated in either zero-IF or low-IF architecture, since image rejection is realized by signal cancellation in two parallel-operating channels (I and Q). However, when multistandard compatibility needs to be addressed, especially for narrowband-wideband-mixed applications, zero-IF and low-IF implementations will encounter obstacles.
Zero-IF receivers are highly appropriate for both wideband or spread-spectrum access standards such as WCDMA [2] and IEEE 802.11a/b/g [3], because the most problematic flicker noise and DC offset are superimposed only on a very small fraction of the desired channel. Thus, without excessive degradation in signal quality, those interferences can be suppressed, for instance, through capacitive coupling. Moreover, the image crosstalk due to unavoidable channel mismatch (namely, I/Q mismatch [4]) will be at a minimum level, as the image is only the upper (or lower) sideband of the one desired.
In contrast, for narrowband standards the removal of flicker noise and DC offset significantly damages the channel spectra since spectrally efficient modulations generally peak at DC. To alleviate those pitfalls, a low-IF architecture [5] was provided for GSM applications by downconverting the desired channel, in frequency, only to the vicinity of DC. Such a solution exhibits comparable integratability as zero-IF receiver and it is therefore extensively used in many narrowband applications today, such as Bluetooth [6], GPS [7], DCS-1800 [8] and IEEE 802.15.4 [9]. Low-IF architectures are efficient mainly because the image-rejection requirement at relatively low IF is still a practical value. However, those architectures are very unfeasible in applications like WCDMA. The adjacent channel selectivity (ACS) test case of WCDMA indicates that, in zero-IF operation, the required image rejection is 25 dB, whereas in low-IF operation the minimum value is 75 dB.
The above architectural boundaries are the main rationales for restricting zero-IF architectures to wideband applications, whereas low-IF architectures are generally designed for narrowband applications. However, today's wireless systems typically are a mixture of narrowband and wideband, such as: WCDMA with GSM [10], or Bluetooth with IEEE 802.11b [11]. To address the demand, a low-IF/zero-IF reconfigurable receiver appears as a new alternative, since the radio-frequency (RF) AFEs of zero-IF and low-IF architectures are theoretically identical [see FIGS. 1(a) and (b)]. For some standards that share the same spectrum, e.g., 2.4-GHz industrial-scientific-medical (ISM) band, the radio can be shared. The remaining inconsistencies predominantly rely on the IF-to-baseband part, so that two dedicated IF-to-baseband chains for zero-IF and low-IF operations are still essential in the past designs shown in [11] and [12]. The solutions provided by those designs, however, inefficiently enlarge the area required and may not be possible if more and more standards need to be complied with, requiring exploration of new techniques to maximize reusability of functional blocks.
In the following paragraphs, the principles of the conventional channel-selection techniques will be presented and their advantages and disadvantages discussed.
Almost all voice- and data-centric standards utilize (or partially utilize) frequency-division multiple-access (FDMA) to divide the entire frequency band into channels for multiple users. The mission of the AFE is to retrieve the sought channel from the air, amplify it and downconvert it from RF to baseband for demodulation. This process is well known in superheterodyne receivers: the sought channel is gradually downconverted and filtered from RF to different IFs, and finally to baseband. On the other hand, image-reject receivers use a series of steps for channel selection, which usually comprise the combination (with possible permutations) of the 3 main blocks, the frequency synthesizer (FS), the local-oscillator (LO) and the channel-select filter (CSF). Depending on the operating frequency (i.e., RF or IF) and movability of the blocks, image-reject receivers can typically be represented by the two alternative architectures discussed in the following paragraphs.
A. Fixed LORF+Varying IF
FIG. 2(a) shows a first type of channel-select architecture [13], where a fixed-frequency RF local oscillator (LORF) is used to perform a large step of RF-to-IF downconversion. After that, the desired channel is extracted at a relatively low-IF value by using a center-frequency-controllable CSF. The sought channel is then downconverted to baseband by way of a further frequency synthesizer and local oscillator. A first advantage of this structure is that it highly relaxes the phase-noise requirement of the RF local oscillator because it is free from locking. A second advantage is due to the fact that channel-select filtering is performed prior to the IF-to-baseband downconversion, so that the operating frequency and the phase-noise requirements of the IF frequency synthesizer and local oscillator can be highly reduced.
However, the main bottleneck of this permutation is that a broadband-tunable filter is required, thus requiring an accurate control of the center frequency. For instance, in a Bluetooth environment, if the entire band (79 channels in total) is downconverted to baseband in the first mixing, a 1-MHz bandpass filter with 79 different center frequencies in a range of 80 MHz (−40 to 40 MHz) is needed. Moreover, the agility of the filter should be high to also allow frequency hopping. With such rigid constraints, it would be very difficult to apply this method in modern applications. However, a special case of this architecture is known for DECT applications, namely a wideband IF double-conversion receiver [14] which employs a fixed-frequency local oscillator cooperating with a wideband lowpass filter in the first downconversion, whereas channel selection is shifted to the second IF. In this way, the operating frequency of the succeeding stages can be reduced. However, this benefit comes at the expense of an increase in the linearity requirements of the wideband lowpass filter to prevent channel-to-channel intermodulation.
B. Varying LORF+Fixed IF
FIG. 2(b) shows a second type of channel-select architecture [15] that uses a RF frequency synthesizer and a LORF to cover all possible channel positions in the RF frequency band of interest. The desired channel is then downconverted to baseband, where only a fixed channel-select filter is needed. This structure is relatively appropriate for state-of-the-art IC designs since current frequency synthesizers (based on PLL architectures) show results at operating frequencies in the GHz range with adequate performance. On the other hand, a fast-settling and broadband-tunable oscillator is much easier to implement than its filter counterpart, and a baseband filter is much simpler and more power-efficient than a bandpass one. The presence of these compromised features confirms the suitability of this type of architecture for almost all kinds of image-reject receivers (e.g., Hartley, low-IF, Weaver and zero-IF) [15].
In summary, the two traditional architectures presented above include variable circuit blocks either at the IF [CSF of FIG. 1(b)] or RF [RF FS & I/Q-LO of FIG. 1(b)], each architecture having its own disadvantages.